The present invention relates to a surface acoustic wave device and a branching filter.
In recent years, rapid development has been made in mobile communication terminals including portable telephones. It is desired that these terminals be particularly lightweight for ease of handling and portability.
To realize compact and lightweight mobile communication terminals, it is essential that electronic parts used therein be also compact and lightweight. For this reason, surface acoustic wave devices, i.e., surface acoustic wave filters, which are advantageous in making them compact and lightweight, are widely used.
As important characteristics required for the surface acoustic wave filters, it is arisen the insertion loss and the spurious response attenuation. The insertion loss affect power consumption of the apparatus, and the lower the loss, the more the battery life is prolonged, so that the battery capacity can be reduced, contributing to the compact and lightweight apparatus. In addition, if high spurious response attenuation can be obtained by a single surface acoustic wave filter, it contributes to the compact and lightweight apparatus.
Conventional surface acoustic wave filters will be described below.
As a filter which satisfies low loss and high attenuation characteristics, it is possible to cite a ladder-type filter.
In the ladder-type filter, a surface acoustic wave resonator connected in series to input and output signal electrodes and a surface acoustic wave resonator connected in parallel thereto are set as one section, and a plurality of such sections are cascade connected. Since these ladder-type filters excel in the low loss and the attenuation of a vicinity of a passband, they are widely used as high-frequency filters of portable telephones.
However, as for the ladder-type filter, if a surface acoustic wave resonator connected in series to input and output signal electrodes and a surface acoustic wave resonator connected in parallel thereto are set as one section, it is difficult to satisfy the required characteristics if only this one section is used. Therefore, it is necessary to use a plurality of sections by cascade connecting them. For example, in a reception-side filter of a branching filter for W-CDMA, it is necessary to use at least seven surface acoustic wave resonators. For that reason, there is a problem in that the chip size increases, and runs counter to the demand for the compact size.
In addition, since the surface acoustic wave resonator needs to be disposed at a position where it does not interfere with the other surface acoustic wave resonators, a long connection line is required to effect predetermined electrical connection. The loss attributable to this connection line becomes such a large problem that it cannot be ignored.